Microwave power limiter utilizing a planar ferrite sphere



May'9, 1967 s. DIXON, JR 3,319,191

MICROWAVE POWER LIMITER UTILIZING A PLANAR FERRITE SPHERE v Filed July8, 1965 Flfeql SOURCE 28 smcuz CRYSTAL 3&Mn- ZnY FERRITE SPHERE 24 dc-ALIGNED WITH 1 X \L EASY AXIS 0F FERRITE INVENTOR,

SAMUEL DIXON JR.

ATTORNEYS United States Patent 3,319,191 MICROWAVE POWER LIMITERUTILIZING A PLANAR FERRITE SPHERE Samuel Dixon, In, Neptune, N.J.,assignor to the United States of America as represented by the Secretaryof the Army Filed July 8, 1965, Ser. No. 470,646 4 Claims. (Cl. 33324.2)

The invention described herein may be manufactured and used by or forthe Government for governmental purposes without the payment of anyroyalty thereon.

This invention relates to wave transmission systems utilizinggyromagnetic elements and more particularly to limiters which provideprotection for receivers in such systems.

It is well known that gyromagnetic mediums such as ferrites arecharacterized by certain unpaired electron spins which respond to atransmitted microwave signal by precessing gyroscopically about the lineof an applied magnetic field. When the frequency of the applied signalis equal to the natural precession frequency of the electron spins, aresonant condition exists under which the electron spins are able toabsorb large amounts of energy from the signal and thereby greatlyattenuate the signal. Assuming a given resonant D.C. magnetic field, itis characteristic of such ferrites that the signal attenuation is verylow below a critical power P but beyond this point the attenuationincreases linearly due to the resonance effects and, as a result, thepower output remainssubstantially constant for input power levels beyondthe critical power level P Power limiting action is thereby achieved.However, for conventional isotropic ferrites, a relatively largermagnetic biasing field is necessary and concomitant large bulky magnetsare usually required.

Recently two groups of ferrites that have very large anisotropy fieldswere discovered and one of these is known as the ferroxplanar group.This group of ferrites has a hexagonal crystal structure with the easyplane of magnetization perpendicular to the C axis, i.e., planaranisotropic, and are hereinafter referred to as planar ferrites. Whensuch planar ferrites are utilized in ferromagnetic resonanceapplications at microwave frequencies, it has been found that themagnetic field required for resonance is greatly reduced when the easyplane of magnetization is aligned parallel to the applied D.C. magneticfield. One such planar ferrite is a crystal having the chemicalcomposition Ba Zn Mn Fe O where x is a numerical value between 0 and1.5, and which is described in copending application Ser. No. 380,103for Method of Growing Single Crystals Containing Manganese, filed July2, 1964. These type crystals are known as Mn-ZnY single crystals where Ydesignates a crystal structure having the general chemical compositionBa Me Fe O as defined on page 185 of Ferrites by Smit and Wijn (1959).These crystals are characterized by a large planar anisotropy field (9600 oersteds) which causes the spin precession to be highly elliptical.This natural elliptical precession endows Mn-ZnY single crystals with avery high parallel susceptibility as compared with other single crystalmaterial, and provides a decrease in the critical power level requiredfor limiting action. Also, the anisotropy field greatly reduces thedirect current magnetic field requirements for an operational limiter.

It is an object of the present invention to provide an improved planarferrite power limiter for operation at microwave frequencies.

It is another object of the present invention to provide a ferrite powerlimiter for operation at microwave fre- 3,319,191 Patented May 9, 1967quencies wherein power limiting may be achieved over a relatively widerange of power variations.

In accordance with the present invention there is provided a powerlimiter which includes a first and second rectangular waveguide joinedtogether in a T-configuration, the first waveguide having input andoutput ports through which a microwave signal propagates. Also includedis a coupling means between the input and output ports for couplingmicrowave signal energy from the first waveguide to the secondwaveguide. The second waveguide is provided with an end wall which iselectrically spaced from the coupling means by A /Z where is thewavelength of the propagated microwave signal. Included further is aplanar ferrite sphere located at said end wall and oriented such thatthe easy plane of magnetization thereof is parallel to the direction ofthe magnetic field produced by the coupled microwave energy, and anexternal unidirectional magnetic field applied parallel. to the easyplane of magnetization of the ferrite sphere.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following descriptiontaken in connection with the accompanying drawing in which:

FIG. 1 is a perspective view, partially cut away, of a power limiterembodying the present invention; and

FIG. 2 is an explanatory curve to illustrate the power output versus thepower input characteristic of the power limiter shown in FIG. 1.

Referring now to FIG. 1 of the drawing, there is shown at 10 arectangular waveguide section adapted to propagate high power microwaveenergy in the conventional rectangular mode in the Z-direction asindicated by the arrow 12. Connected to one end of waveguide 10 is asource 14 of microwave energy having a relatively high power level as,for example, a high power transmitter. Connected to the other end ofguide 10 is a load 16 to which it is desired to introduce microwavesignals at, but not exceeding, one given power level. Attached to oneside wall of waveguide section 10 is a resonant rectangular waveguidecavity section 18. Resonant cavity section 18 comprises a section ofrectangular waveguide having broad walls 22 and 24, narrow side walls 26and 28 and end walls 30 and 32, with end wall 30 being affixed to theside wall of waveguide section 10 and an integral part thereof. A mutualcouplying slot 3-3 is provided in end wall 30 so that electromagneticenergy is coupled from waveguide section 10 into resonator 18 and thelength of resonator 18 is made one-half wavelength of the operatingfrequency. The microwave energy coupling from waveguide 10 to resonator18 is such that a very intense magnetic field is established 'very closeto the end wall 32. of resonator 18 as indicated by the dashed line 34.Attached to end wall 32 substantially at the center thereof withinresonator 18 is a sphere 38 of the abovementioned planar single crystalMn-ZnY ferrite with the easy plane of magnetization of the planar Mn-ZnYsphere 38 oriented parallel to the direction of the RF magnetic field 38resulting from the propagated microwave energy as hereina-bovedescribed. A unidirectional magnetic biasing field H indicated by thearrow 40 is applied parallel to the direction of the RF magnetic fieldclose to end Wall 32 and in the easy plane of magnetization of theplanar Mn-ZnY sphere 18. If, as shown in FIG. 1, the electromagneticwave propagated energy is in the Z-direction, and with the wave front inthe x-y plane, then the RP. magnetic field, the easy plane ofmagnetization and the applied unidirectional magnetic biasing field willbe parallel to the Z-direction. With such an arrangement the naturalelliptical precession motion of the Mn-ZnY sphere 18 induces anoscillatory magnetic moment parallel to the Z-direction. With the RFmagnetic field being of a frequency equal to that of the oscillatoryZ-magnetic moment, the precesssional motion is further excited, providedof course, that the RF field is of sufficient magnitude so that energyis pumped into the spin system faster than it is lost due to relaxationprocesses.

In operation, at low power levels, there is no interaction between theRF energy and the Mn-ZnY sphere 18 so that the cavity reflects a shortat the plane of the coupling iris 33. Thus, at relatively low inputpower, the electromagnetic wave energy propagates with very little loss.At the limiting power level P (FIG. 2), absorption of energy occurs inthe Mn-ZnY sphere 18 due to the excitation of the spin waves. Thus, itwill be seen that the power output remains substantially constant atlevel P for large increases in the power input.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is therefore aimedin the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What is claimed is:

1. A power limiter comprising a first and second rectangular waveguidejoined together in a T-configuration, said first waveguide having inputand output ports through which a microwave signal propagates,

coupling means between said ports for coupling said microwave signalenergy from said first waveguide to said second waveguide,

said second waveguide having an end wall electrically spaced from saidcoupling means by x 2 where A is the wavelength of said signal,

a planar ferrite sphere located at said end wall and oriented such thatthe easy plane of magnetization of said sphere is parallel to thedirection of the magnetic field produced by the coupled microwaveenergy,

and an external uni-directional magnetic field applied parallel to saideasy plane of magnetization of said ferrite sphere.

Z. The power limiter in accordance with claim 1 wherein said ferritesphere comprises an Mn-ZnY planar ferrite.

3. A power limiter comprising a first rectangular waveguide havingopposing narrow and broad walls and adapted .to propagate a microwavesignal in the TE mode,

a secon drectangular waveguide orthogonally positioned with respect to anarrow wall of said first waveguide, and terminated by an end wallspaced X /Z wavelength therefrom, where is the wavelength of saidmicrowave signal,

means for coupling said microwave signal from said first waveguide tosaid second waveguide whereby there is established a RF magnetic fieldparallel to and adjacent said end Wall, and in the direction ofpropagation of said microwave signal,

a planar ferrite sphere within said second waveguide and attached tosaid end wall, said sphere being oriented such that the easy plane ofmagnetization of said sphere is parallel to said RF magnetic field atsaid end wall,

and an external unidirectional magnetic field applied parallel to saideasy plane of magnetization and said RF magnetic field.

4. The power limiter in accordance with claim 3 wherein said ferritesphere comprises an Mn-ZnY planar ferrite.

References Cited by the Examiner UNITED STATES PATENTS 2,946,753 7/1960Jonker et al 25262.5 3,113,278 12/1963 Okwit 33324.2 3,131,366 4/1964Dixon 33324.2

OTHER REFERENCES Philips Techanical Review, vol. 18, 1956/57, No. 6,Nov. 30, 1956, pp. -154.

HERMAN KARL SAALBACH, Primary Examiner.

P. L. GENSLER, Assistant Examiner.

1. A POWER LIMITER COMPRISING A FIRST AND SECOND RECTANGULAR WAVEGUIDEJOINED TOGETHER IN A T-CONFIGURATION, SAID FIRST WAVEGUIDE HAVING INPUTAND OUTPUT PORTS THROUGH WHICH A MICROWAVE SIGNAL PROPAGATES, COUPLINGMEANS BETWEEN SAID PORTS FOR COUPLING SAID MICROWAVE SIGNAL ENERGY FROMSAID FIRST WAVEGUIDE TO SAID SECOND WAVEGUIDE, SAID SECOND WAVEGUIDEHAVING AN END WALL ELECTRICALLY SPACED FROM SAID COUPLING MEANS BY $G/2WHERE $G IS THE WAVELENGTH OF SAID SIGNAL, A PLANAR FERRITE SPHERELOCATED AT SAID END WALL AND ORIENTED SUCH THAT THE EASY PLANE OFMAGNETIZATION OF SAID SPHERE IS PARALLEL TO THE DIRECTION OF THEMAGNETIC FIELD PRODUCED BY THE COUPLED MICROWAVE ENERGY, AND AN EXTERNALUNIDIRECTIONAL MAGNETIC FIELD APPLIED PARALLEL TO SAID EASY PLANE OFMAGNETIZATION OF SAID FERRITE SPHERE.